1. Field of the Invention
The present invention relates to an active matrix organic electroluminescence device and a manufacturing method of the same.
2. Discussion of the Related Art
In general, an organic electroluminescence device emits light by injecting an electron from a cathode electrode and a hole from an anode electrode into an emissive layer, combining the electron with the hole, generating an exciton, and transiting the exciton from an excited state to a ground state. The organic electroluminescence device has a small size and light weight because it does not require additional light sources.
The organic electroluminescence device is categorized into a passive matrix type and an active matrix type according to a driving method.
The passive matrix type organic electroluminescence device has a simple structure and is manufactured through a simple process. However, the passive matrix type organic electroluminescence device has high power consumption and is difficult to make to have a large area. Additionally, in the passive matrix type organic electroluminescence device, an aperture ratio decreases according to the increasing number of electro lines.
Therefore, the passive matrix type organic electroluminescence device is widely used as a small size display device. On the other hand, the active matrix type organic electroluminescence device is widely used as a large size display device.
An active matrix type organic electroluminescence device according to the related art will be described hereinafter more in detail.
FIG. 1 is an equivalent circuit diagram for a pixel of an active matrix type organic electroluminescence device in the related art. In FIG. 1, a gate line 12 is formed horizontally in the context of the figure on a substrate 10. A power line 9 is formed parallel to the gate line 12. A data line 14 is formed vertically in the context of the figure and crosses the gate line 12 and the power line 9. The gate line 12 and the data line 14 define a pixel region. A pixel includes two thin film transistors (TFTs) “T1” and “T2,” an electro-luminescent diode “EL,” and a storage capacitor “C.” The TFTs are composed of a switching TFT “T1” and a driving TFT “T2.” Each TFT includes a gate electrode, an active layer, and source and drain electrodes. The switching TFT “T1” is formed at the crossing of the gate line 12 and the data line 14. The gate electrode of the switching TFT “T1” is electrically connected to the gate line 12 and the source electrode of the switching TFT “T1” is electrically connected to the data line 14. The gate electrode of the driving TFT “T2” is electrically connected to the drain electrode of the switching TFT “T1 ,” the source electrode of the driving TFT “T2” is electrically connected to the power line 9, and the drain electrode of the driving TFT “T2” is electrically connected to one end of the electroluminescent diode “EL,” that is, an anode electrode. The other end of the electroluminescent diode “EL,” i.e., cathode electrode, is ground. One end of the storage capacitor “C” is electrically connected to the gate electrode of the driving TFT “T2 ” and the other end of the storage capacitor “C” is also grounded.
When the driving TFT “T2” turns on by a signal through the switching TFT “T1 ” from the data line 14, a signal from the power line 9 is transmitted to the electroluminescent diode “EL” through the driving TFT “T2,” and light is emitted from the luminescent diode “EL.”
Although the switching TFT “T1” turns off, the storage capacitor “C,” which is connected to the switching TFT “T1” and the driving TFT “T2,” maintains the signal from the data line 14 until the next signal is transmitted. Therefore, signal loss is compensated. However, there may be signal loss due to an increasing resistance of the electroluminescent diode “EL” with the lapse of time.
To compensate the signal loss, an organic electroluminescence device having four TFTs has been proposed. However, formation of the four TFTs reduces the production rate of the devices.
Meanwhile, p-type polycrystalline silicon is widely used as an active layer of the switching TFT “T1 ” and the driving TFT “T2.” Polycrystalline silicon may be formed by a crystallization method using a laser, but the polycrystalline silicon has poor uniformity. Moreover, a manufacturing process of the polycrystalline silicon is complicated and requires a lot of time due to deposition of an amorphous silicon layer, irradiation of a laser beam, and crystallization of the amorphous silicon layer.
Accordingly, in manufacturing the four TFTs including polycrystalline silicon, production costs of the device are increased due to the increasing number of manufacturing processes. Besides, the uniformity of image may be reduced.
To solve the problem, an organic electroluminescence device including an amorphous silicon TFT, which has amorphous silicon as an active layer, is shown in Korean Patent Publication No. 2001-0027787. The amorphous silicon TFT is p-type in which holes are the major carrier. However, the p-type amorphous silicon TFT is difficult to manufacture and has lower field effect mobility and lower on-current than an n-type amorphous silicon TFT in which electrons are the major carrier.
FIG. 2 is a circuit diagram of an active matrix type organic electro-luminescence device having an n-type amorphous silicon TFT in the related art and corresponds to a region “A” of FIG. 1.
In FIG. 2, the driving TFT “T2” includes n-type amorphous silicon as the active layer. The source electrode “S2” of the driving TFT “T2,” which is a source of the major carrier, i.e., electrons, is connected to the anode electrode of the luminescent diode “EL” and the drain electrode “D2” of the driving TFT “T2” is connected to the power line 9. The cathode electrode of the luminescent diode “EL” is grounded. The storage capacitor “C” is connected to the gate electrode “G2” of the driving TFT “T2” and the grounded cathode electrode of the luminescent diode “EL.”
In the driving TFT “T2,” a current between the drain electrode “D2” and the source electrode “S2” is given byI=μnCi (W/L)[(VGS−VTH)VDS−(VDS2/2)],where μn is a field effect mobility of the driving TFT “T2,” Ci is a gate capacitance of the driving TFT “T2,” W is a width of a channel of the driving TFT “T2,” L is a length of the channel, VGS is a voltage between the gate electrode “G2” and the source electrode “S2,” VTH is a threshold voltage, and VDS is a voltage between the drain electrode “D2” and the source electrode “S2.”
The driving TFT “T2” is used in a saturation region, where the current “I” is constant to the increasing voltage “VDS,” so as to supply a regular current to the electro-luminescent diode “EL.” In the saturation region, the voltage VDS should be larger than a difference between the voltage VGS and the threshold voltage VTH, i.e., VDS>VGS−VTH.
By the way, electrical resistance of the electroluminescent diode “EL” increases as time goes by, and a voltage “VEL” of the electroluminescent diode “EL” changes. Then, since a voltage at the source electrode “S2” changes, the voltages “VGS” and “VDS”vary, too. Here, the voltage “VGS” and the voltage “VDS ” decrease. Therefore, the voltages “VGS” and “VDS” should be changed in order to provide the electroluminescent diode “EL” with an equal amount of current before.
Moreover, because the anode electrode of the electroluminescent diode “EL” is connected to the source electrode “S2” of the driving TFT “T2,” which is the source of electron as stated above, more voltage should be applied to the gate electrode “G2” through the data line 14 of FIG. 1. Accordingly, new driving integrated circuit, which is electrically connected to the data line 14, is required.